This is a popular idea, but it’s mostly false. The main reasons immigrant groups today are unusually successful is the same as it has been throughout American history: with a few exceptions, immigrant groups are selected from the higher socioeconomic strata of their home countries. This is partly because mobile people tend to be wealthier and more educated in the first place, but it’s also a function of our immigration restrictions.

A recent book by Leah Boustan and Ran Abramitzy covers this topic over the past century. Research by Ed Lazear covers the part about how current country-specific numerical limits function to select higher-educated groups.